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PJMS- Volume 3 Number 1: January-June 2013 Review Article Pulmonary function tests in clinical practice: Importance, requirements and limitations Choudhary Sumer1 Abstract: 1 Associate Professor, Department of Pulmonary Medicine, NKP SIMS&RC, Digdoh Hills,Hingna Road, Nagpur -440019. [email protected] Pulmonary function tests have progressed from initially used water seal types to modern era electronic computerized versions. The newer software are comparatively easier to operate and less time consuming. They are patient friendly and easier to understand. However there are limitations to pulmonary function tests, as the pattern of abnormality indicates type of problem however they do not provide anatomic diagnosis. Battery of tests is available which help in evaluation of different aspects of pulmonary function. It is possible to monitor the progression of disease and effect of management. However no single test can evaluate all aspects of pulmonary function. Pulmonary function test can be carried out at bedside in critically ill patients with the help of portable spirometers in addition to the routine clinical tests. Commonly performed pulmonary function tests are dynamic studies –pre and post bronchodilator tests, evaluation of lung volumes with body plethysmography, nitrogen washout or helium dilution methods, diffusion capacity of the lung carbon monoxide by single breath analysis, arterial blood gases and pulse oximetry. Other commonly performed tests are maximal expiratory and inspiratory pressure, exercise induced(Treadmill)or allergen induced bronchoprovocative tests, shunt studies and Dead space measurements. If the pulmonary function tests are done with quality assurance, validation of the equipment, proper technique, reference values and applying right ethnic correction factors, the data generated are most of the times accurate and reproducible. Keywords: Spirometry, Static lung volume, Pulmonary function, Diffusion capacity, Plethysmography. Objectives: 1) To understand the need of pulmonary function test as a routine in a pulmonary clinic. 2) To review the methods to evaluate the lung function's. 3) To appreciate that pulmonary function test interpretation with relevant patient information has greater clinical meaning. 4) To recognize the limitation of pulmonary function tests. 5) To know the mistakes and pitfalls during pulmonary function test and how to avoid them. Introduction: 7) 8) Detection of the precipitating factors. Smokers who are at higher risk of developing airway disease. 9) Those who are at occupational risk of developing airway disease such as coal miners, farmers etc. The results of pulmonary function test of a given individual are compared with those obtained from a normal population of comparable height, age and gender. The test is considered abnormal if it falls outside the range based on standard error of the estimate in which 95% lies (1). Measuring the lung function is now a routine clinical tool in OPD for a practicing pulmonologist as it is not possible to detect conduction disorders in heart on skiagram, similarly one can't measure functional/physiological changes of lung on X-Ray for which pulmonary function test is mandatory. The treating physician should know about the physiology and pathology of lung in health and disease for proper management of patients. Instruments Used for Measuring Pulmonary Function Peak Flow Meter: This is a handheld device routinely used in pulmonary OPD. There are different scales used. The scale closest to the ethnic and race of the region should be used. It measures peak expiratory flow rate. Inspiratory flow rate can be also measured in some devices. Spirometry: Initially there were two types of spirometry used. They were mainly: 1) Closed: a) Wet-Classical bell spirometer with water seal which was commonly used prior to electronic pulmonary test. b) Dry- They are bellow volume, rolling seal, electronic spirometers. Pulmonary Function Tests provide 1) 2) 3) 4) 5) 6) Objective evidence of deranged lung function caused by underlying disease involving the bronchial tree, pulmonary vasculature and chest bellows. Help to identify the underlying cause either obstructive or restrictive. Helpful in preoperative evaluation. Helpful in assessing progress of the diseases. Helpful in monitoring the efficacy of treatment. Evaluation of disability. A spirometer, including the waterless, rolling seal type and Stead-Wells water seal type is an instrument that directly measures the volume of air displaced or measures airflow by a flow-sensing device, such as a pneumotachometer or a tube 7 PJMS- Volume 3 Number 1: January-June 2013 Review Article containing a fixed resistance to flow (2). Today, most clinical pulmonary function testing laboratories use a microprocessordriven pneumotachometer to measure air flow directly and then to mathematically derive volume. P1-resting mouth pressure V-FRC P2-pressure on inspiration rV-rise in lung volume on inspiration Methods and Means to Evaluate Functions of lung Types of Spirometers (3) With the help of the above mentioned devices one can measure static and dynamic lung volumes. The static mechanical properties of the lung are: l Static compliance of the lungs. l Static compliance of the chest wall. l Elastic properties of the respiratory system as a whole. l Respiratory muscle strength. The dynamic mechanical properties of the respiratory system are: l Forced vital capacity. l Flow volume curves. l Maximal voluntary ventilation. Volume Bellows Rolling seal Water Dry Flow sensing (Pneumotach) Fleisch Screen Hot- wire Computerized Spirometry: Now-a-days most commonly used method. It is comparatively easier to use than the older versions, is doctor and patient friendly device with almost accurate results. Bedside Clinical Tests There are certain bedside tests which can help a clinician in assessing the lung function. These are: Pnemotactograph: This is another instrument very popularly used for testing airflow. This device measures instant respiratory airflow. Differential pressure transducer or turbine or volume transducer are used of which differential pressure flow transducer is commonly used. In this a sensitive manometer detects the pressure drop across the slight resistance, in the air stream Ultrasonic flow meters and the hot wire anemometer is the other types used (3,4). Body Plethysmography: This instrument was first introduced by DuBois and colleague's in 1956. The word plethysmography is a Greek word means “enlargement”(5). It measures both absolute lung volume and airway resistance. There are many types available. Three different types of body plethysmography are used. l Pressure Plethysmography l Volume Plethysmography l Pressure Corrected Flow Plethysmography Most commonly used is constant-volume plethysmography. This method provides more accurate measure of thoracic gas volume in patients with lung disease associated with gas trapping. Plethysmography is a complex test. Proper calibration of transducers and frequency response, thermal stability and leaks should be properly checked (6-8). The patient is seated comfortably, told to relax remove dentures and support cheek. The door is closed and the thermal transients to stabilize. After establishing a stable baseline representing Functional Residual Capacity (FRC), airway is occluded by closing the valve at end expiration and the patient is instructed gentle pants at a frequency of 1Hz. FRC is calculated as: P1V=P2(v+rV) 8 1) Snider Test: The ability to blow a match held at 15 cm from mouth. This correlates with maximum voluntary ventilation of 60 liter/min or FEV1 of 1.6 litres. Same test done with a match held at 8 cm correlates with maximum voluntary ventilation of 40 liters/min (9). 2) Forced Expiratory Time: It is measured over the trachea. It is measured from the beginning to the end of the maximal expiration from total lung capacity. Most normal subjects have a forced expiratory time of 4 seconds and patients with obstruction have more than 6 seconds (10, 11). Now-a-days portable spirometers are available by which lung function test can be done at the bedside. Other clinical tests are tracheal descent with inspiration, inspiratory excavation of supraclavicular and suprasternal fossae and paradoxical inward movement of the lower ribs during inspiration. These tests are good to detect qualitative abnormalities but lack precision of quantitative measurement. Following measurements are done in lung function: 1. Airway Function: This includes following: 1) 2. 3. 4. Spirometry : Vital Capacity (VC), Forced Expiratory Volume in 1 sec (FEV1), Forced Vital Capacity (FVC), FEV1/FVC, Forced expiratory flow 25%-75%. Flow Volume loops. Peak Expiratory Flow Rate. Airway resistance by body plethysmography. 2) Lung Volumes: Lung volumes are commonly measured by: PJMS- Volume 3 Number 1: January-June 2013 Review Article 1. 2. 3. Spirometry Gas Dilution Technique Plethysmography are highest around noon and lowest during early morning. In normal persons FEV1 increase by 0.15l in the morning and decrease by 0.05l in the afternoon. In asthmatics, lung function parameters are lower in the morning (Morning Dip). Circadian variations have also been documented for airway resistance, functional residual capacity, total lung capacity and residual volume. Lung volumes commonly measured are tidal volume, total lung capacity, functional residual capacity and residual volume. 3) Diffusion: It is the ability to transfer oxygen across the alveolar –capillary interface is diffusion capacity. 4) Respiratory Muscle Function: Peak inspiratory and peak expiratory maximal mouth pressure is simple test to evaluate respiratory muscle strength. Variation in measured lung function between individuals can largely be attributed to height, weight, sex, age, health statesman to some extent, race. Around 27%of interindividual variation remains unexplained (16).Young men have continuous increase in lung function till reaching adult height as compared to young women. After age of 30 years, a non smoking individual usually loses 25 to 30 ml of FEV1 every year. Measured spirometric values also differ between races. Spirometry The American Thoracic Society standardization guidelines for acceptability and reproducibility criteria are shown in (12). A well-trained pulmonary function technician usually coaches the patient through the session until the demonstrated reproducibility of key parameters suggests the results represent the best possible measure of lung function at that time. Spirometry is the most commonly used test to measure lung functions. Modern spirometer utilizing computer technology offer distinct advantages over earlier generation instruments. Spirometers can be programmed to detect and flag technique related factors that effect results, including cough, late peak flows, premature effort termination and variation in maneuvers (12, 13). Before starting the tests, technician should ascertain that the subject has understood the instructions. One should observe following precautions: l l l l l l Acceptability and Reproducibility Criteria for Spirograms Acceptability Criteria(12): Patient should avoid wearing tight clothes which may restrict the chest movements and abdominal expansion and be instructed against smoking, alcohol consumption, vigorous exercises, or eating large meals 2 to 4 hours prior to test. Data gathered prior to testing include patient age, height, weight, and time of day. Transmission of infection should be avoided by strictly adhering to hygiene and infection control measures. Unobstructed mouth piece: Remove prosthetic loose dentures if any, put mouth piece over the tongue. Maximum inspiration. Smooth continuous expiration with maximal effort. Body position has a significant impact on spirometry, especially FVC and vital capacity. The values are 8% and 2% lower, respectively, in supine and sitting position, compared to standing. This is important in comparison studies and obese persons. Increased peak expiratory flow is seen in hyper extension of the neck due to elongation and stiffening of the trachea. Flexion of the neck decreases peak flow and increases airway resistance (14, 15). 1. Free from artifacts l Cough or glottis closure during the first second of exhalation Early termination or cutoff Variable effort Leak Obstructed mouthpiece l l l l 2. l l 3. l l l Good start Extrapolated volume is <5% of FVC or 0.15 L, whichever is greater Or Time to PEF is <120 ms (optional until further information is available) Satisfactory exhalation 6sec of exhalation and/or a plateau in the volume-time curve or Reasonable duration or a plateau in the volume-time curve or The subject cannot or should not continue to exhale Repeatability Criteria In certain patients, repeated force tests may trigger bronchospasm resulting in decline of FVC and FEV1. A diagnosis of reactive airway disease should be considered, technical factors should not be assumed to explain this observation. Lung function has a diurnal pattern. Peak flows After three acceptable spirograms have been obtained, apply the following tests. l l 9 Are the two largest FVCs within 0.2L of each other? Are the two largest FEV1s within 0.2L of each other? PJMS- Volume 3 Number 1: January-June 2013 Review Article l l l If both of these criteria are met, the test session may be concluded. If both of these criteria are not met, continue testing until: Both of the criteria are met with analysis of additional acceptable spirograms or A total of eight tests have been performed or Save a minimum of three best maneuvers Static Lung Volumes Static lung volumes are determined using methods in which airflow velocity does not play a role. The sums of two or more lung volume subdivision constitute a lung capacity and are expressed in liters at body temperature. Functional Residual Capacity (FRC), the volume of gas present in the lung at end expiration during tidal breathing, is a key component in the measurement of lung volumes. FRC can be assessed by following methods: alveolus to the hemoglobin binding site, hence the term transfer factor. It is commonly determined by the use of CO, as its uptake is easy to measure and it follows the same diffusion pathways that of oxygen. The basic equation used to calculate DLCO is: DLCO = Vco/PACO-PCCO Where: Vco is rate of disappearance of CO PACO is alveolar concentration of CO PCCO partial pressure of CO in blood To ensure consistency following precaution should be taken. 1. The patient should be comfortably seated upright and fully co operative. 2. The patient should not have recent strenuous exercise or meal within 2 hours of prior to test. 3. The patient should refrain from smoking and alcohol intake. Nitrogen Washout Method (open- circuit method using N2) This technique was introduced in 1940 by Darling, Cournand, and Richards. In this method the patient breathes 100% oxygen for 7 minutes and during this period Nitrogen concentration in expired air is measured. When the N2 level falls to zero all N2 present in the lungs at the beginning of the test has been washed out. The total volume of gas expired and N2 in the expired gas is measured (17). FRC= (volume of N2washed out)-(N2 tissue extraction) /initial –finalN2 concentration. N2 tissue extraction is calculated as (Body Surface Area x 96.5) +35/ 0.8 l Helium Dilution Method (closed – circuit method u s i n g helium) This is a closed circuit method. The system is readied by adding 2L of air and sufficient helium to achieve concentration of approximately 10% helium. The patient rebreathes from the closed circuit. The CO2 is absorbed by absorbent while O2 is added through valve. As the helium mixes with the air in the lungs its concentration falls. Stabilization of the helium concentration indicated by a rate of change of less than 0.02 percent over 30 sec interval, indicates helium concentration has equilibrated through the lung breathing circuit system. The test usually ends in 7 minutes. FRC=Initial He %-Final He %/Final He % (5, 18,19). l Body Plethysmography As described previously. Imaging techniques and sulphur hexafluoride washout are not routinely used. The maneuver begins with unforced exhalation to residual volume. A good effort is when the inspiration is rapid to ensure instantaneous lung filling and the inspired volume is 85 to 90% of vital capacity. Slower lung fillings reduces CO uptake and a sub maximal effort from residual volume reduces alveolar volume and produces low DLCO. A breath hold time of 10 to 12 s is standard (2,20-24). Measurement of Arterial Blood Gasses Oximetry arterial blood gasses and pH analysis are done to evaluate the ventilatory, acid base status, oxygen carrying capacity, and intrapulmonary shunt (25). Factors Affecting Outcome of Pulmonary Function Test The factors affecting lung functions can be broadly described as analytical and non-analytical factors (20, 26-27). Non-Analytical Factors Diffusion Capacity (2) The non-analytical factors are mostly patient related factors. Anxiety related to the procedure, smoking just before the test, underlying pain, wearing tight clothing, certain medication, time of the test, having meals before the test may alter the actual findings in a particular patient. Diffusion Capacity: It implies the maximum transfer ability of the lung and is governed by its structural and functional properties. The gas has to travel through several barriers as it moves from the 10 PJMS- Volume 3 Number 1: January-June 2013 Review Article The major non-analytical factor leading to misinterpretation is ethnic correction scale used. Most of the earlier versions of electronic spirometers have correction scale according to European standard, whose height weight and age factor doesn't correspond to our region. Now-a-days Spirometers are available in which we can manually enter ethnic correction factor according to the place where the test is being carried out. 7. 8. 9. Analytical Factors There are many opportunities of error in doing lung function test. This may be equipment related, technique employed or calculations done. In the beginning there was no proper standardization of the different test carried out. In the recent era a global standardization of the method, calculation technique and precaution to be taken have been formulized which if followed would certainly give more accurate results. 10. 11) Conclusion : 12) There is no doubt that pulmonary function tests play an important role in diagnosis, management, and followup in patients at risk and those who are having lung disease. Since the invention of spirometry, when water seal types spirometer where used to the recent times, has passed through various generation to the present computerized electronic spirometer. The presently used spirometers have ethnic correction factor according to the region and can also be fed manually by the treating pulmonologist. The newer generation spirometer are simpler to use and both patient and doctor friendly. The new software used are self explanatory and the patient either adult or children can easily understand the type of test to be done. 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